Motivated by the above considerations, we study an IRS-assisted WPCN where an IRS is deployed to assist the TDMA-based DL WPT and UL WIT between an HAP and multiple devices, as shown in Fig. REF . Our objective is to maximize the weighted sum throughput of all devices by jointly optimizing the resource allocation and IRS phase shifts (i.e., passive beamforming). It is worth noting that unlike traditional WPCNs where the channels of all devices are generally random and remain static throughout each channel coherence block [1]}, we are able to proactively generate favourable time-varying channels by properly designing the IRS phase-shift vectors over different time slots, which thus enhances the multiuser diversity over time and also allows more flexible resource allocation. The main contributions of the paper are summarized as follows.
